The invention relates generally to froth flotation for separating out minerals from ground-up ore and, more particularly, to the recovery of minerals from the relatively fine particle froth flotation feed, commonly known as xe2x80x9cslimes,xe2x80x9d which conventionally is discarded.
Froth flotation is a well-known process used to separate minerals, ground up into particles and suspended in or otherwise carried by a liquid, by attaching the mineral particles to gas bubbles to provide selective levitation of the solid particles into a froth. Conventionally the liquid is water. Selective levitation is accomplished by conditioning a flotation feed in the form of a slurry with various flotation reagents that selectively coat the particle surfaces of various minerals. The surface coating allows for either air bubble attachment to individual particles or prevents air bubble attachment, depending on the specific reagents used in conditioning and subsequent flotation. In some cases the desired mineral particles are carried upward into the froth and collected as product, leaving other material to settle as tails, which can be waste. In other cases, undesired particles are carried upward into the froth and discarded as waste, leaving desired mineral particles to settle as tails, which is collected as product.
For effective separation, it is essential that the particles be discrete particles of the individual minerals. To promote the most efficient and selective response to the flotation reagents utilized, it is also important that the particles have clean non-contaminated surfaces. (However, not all froth flotation facilities employ scrubbers.)
To produce discrete mineral particles, ore is crushed and ground to nominally 1 mm diameter and finer particle size for flotation feed. This crushing and grinding produce some material as fine as 0.001 mm. Normal flotation practices are performed over a particle size range of the feed determined to yield the most efficient, cost-effective and quality-acceptable flotation product. The following table lists the desired smallest size particle for flotation feed for various minerals, which may be viewed as a minimum particle size cut off point, as given by Crozier in Flotation, Theory, Reagents and Ore Testing.
These minerals are listed as examples only, and the list above is not all-inclusive. The majority of minerals recovered by froth flotation are currently processed at a minimum particle size cut-off point.
Relatively fine particles smaller than the minimum particle size, referred to as fines or slimes, interfere with efficient froth flotation. Under current practice slimes are therefore discarded, even though they contain significant quantities of usable minerals. For the minerals listed above, approximately 10%-20% of the flotation feed typically is finer than the minimum particle size cut-off point.
To provide mineral particles that have clean non-contaminated surfaces, scrubbing processes are employed in some froth flotation facilities. As an example, a conventional attrition scrubber takes the form of a tub into which a slurry is loaded. The slurry typically contains approximately 70% solids by weight in the form of particles to be cleaned, and is conditioned with cleaning reagents such as NaOH, H2SO4, sodium silicate, HCl and sodium hexametaphosphate, depending upon the particular minerals involved. Reagents serve cleaning, dispersion and conditioning functions. A rotating vertical shaft extends into the tub, and carries impellers which are angled so as to alternately push the slurry up and down. The particles rub against each other to effect cleaning, aided by the cleaning reagents.
It is therefore seen to be desirable to efficiently recover minerals from conventionally discarded fine froth flotation feed (slimes).
In an exemplary embodiment, the slimes are scrubbed in the presence of attrition media, and subsequently processed by froth flotation. The attrition media is removed either before or after froth flotation of the slimes.